This invention relates to variable coupling optical waveguide couplers.
One example of a suitable double heterostructure is that of the GaAlAs -- GaAs -- GaAlAs structure developed for injection laser manufacture. Typically the refractive index difference between the middle and flanking layers is great enough to provide relatively strong guiding so that the middle layer can be reduced in thickness. This guiding is in a direction normal to the planes of the heterojunctions.
In order to fabricate a waveguide, guiding is also required in a direction lying in the planes of the heterojunctions. This can be achieved by known techniques, such as impurity diffusion and proton bombardment, used to alter locally the electron concentration within the middle layer. The light is constrained to propagate in the region of lower electron concentration, but in this case the refractive index difference that is produced is normally smaller, and hence the width of the waveguide is made larger than its height, and may be typically within the range 2 to 8 .mu.m.
The coupling between a pair of optical waveguides that are in fixed spatial relationship to one another is accomplished by altering the guiding properties by changing the refractive index. This change may be brought about electrically in a direct manner making use of the electro-optic effect. In semiconductive material having a pn junction there is a second way of changing the refractive index electrically. The application of a reverse bias to a pn junction produces a change of refractive index on the n-type side of the junction in the depletion region as a result of the extraction of electrons from that region. Where this extraction of electrons effect is present, the electro-optic effect will also be present since there is an electric field across the depletion region.
GaAs is a sensitive electro-optic material, the interaction being in fact stronger than in materials such as LiNbO.sub.3 which already form the basis of effective optical switches. The p-n junction is a convenient structure across which to apply the electro-optic field. It is then not necessary to produce high resistivity GaAs (for reducing current flow) since the p-n junction does this automatically when reverse biassed. The field appears across the depletion region. The only condition to be met in the semiconductor doping level is that the carrier concentration should be low enough to enable the depletion layer under reverse bias to extend across an appreciable proportion of the middle optical guiding layer of the semiconductor sandwich.
The electro-optic effect in GaAs is such that the application of an electric field across a p-n junction produces in general a change in refractive index for light propagating in the plane of the junction. The precise effect depends on the crystallographic orientation of the junction, the polarization of the light and the direction in which it propagates in the junction plane. With the junction in the (100) plane the maximum change in refractive index occurs for a mode with its electric field polarized in the junction plane and propagating in any of the relevant (110) directions. There is zero change for propagation in the (100) directions and there is zero effect for all relevant directions of propagation of a mode with its electric field polarized perpendicular to the (100) plane. With the junction in the (111) plane the effect is independent of the direction of propagation of the modes in the junction plane but is twice as great for a mode with its electric field polarized in the plane of the junction as for the complementary polarization, and of opposite sign. It will be noticed that for neither of these orientations, or for any other possible orientation, is it possible to obtain the same effect for modes of both polarizations.
The maximum usable change of refractive index, for modes polarized in the plane of the junction, is of the order of 0.0005 for a reverse bias approaching breakdown. The electric field extends in the direction normal to the p-n junction to a distance approximately equal to the thickness of the depletion layer. For this distance to be about equal to the thickness of the light guide (not less than 0.2 .mu.m) at the highest voltage that can be applied, the doping should be less than about 3 .times. 10.sup.17 cm.sup..sup.-3. The voltage required is then about 15 volts.
In addition to its effect on the electro-optic interaction the field also changes the refractive index of the region close to the p-n junction by removing the free carriers. At the highest doping level suggested above of 3 .times. 10.sup.17 cm.sup..sup.-3 removal of electrons also (by coincidence) perturbs the refractive index by approximately the same value of 0.0005. In this case the refractive index change is isotropic, and the field increases the value of the refractive index. Hence under certain conditions, but not at low doping levels, depletion of carriers by an applied field can be used for switching purposes almost as effectively as the electro-optic effect.
A change in refractive index of 0.0005 is very significant compared with the refractive index variation in the plane of the junction associated with the waveguide structure (and induced by ion beam processing or diffusion of impurities). The refractive index step that is required for single mode propagation in the plane of the junction decreases as the inverse square of the guide width and is typically 0.0005 for a guide width of 4 .mu.m. Hence the perturbation of refractive index produced by the electro-optic effect or by the carrier depletion effect can be sufficient either to confine light to a guide width of 4 .mu.m without any existing refractive index variation, or conversely, to destroy an already existing guiding effect of such a magnitude. This can be immediately applied to switching applications since it enables alternative connections to be made or destroyed between any number of open ended input guides and output guides. If the effect is strong enough to be applied to open ended guides in this fashion then it can also be applied to the less demanding situation of modifying the coupling between two guides lying alongside one another to make an alternative and more sensitive form of switch.
Two methods are possible for using the electro-optic interaction or the electron depletion effect to give this variation of coupling. In one method the normal electric field is applied across the region in between the two guides, and in the other it is applied across one of the guides. In the first case the coupling is directly affected by the field and in the second case it is affected indirectly as a result of the consequent interference with the phase coherence between the two guides.
Different forms of construction are used for the two methods. The layout is dictated by the requirement firstly that a p-n junction must be provided at the point where the field is to be applied, and secondly that the guides themselves must be composed of either semi-insulating material or p-type material, while the regions on either side must be n-type material. This second requirement gives the appropriate relative values of the refractive index for optical guiding in a direction lying in the planes of the heterojunctions while optical guiding in the direction normal to the planes of the heterojunctions is provided by the changes of refractive index at the heterojunctions themselves.
It is an object of the present invention to provide an optical waveguide coupler wherein a p-n junction is provided at the point of applied field and wherein the guides themselves are composed of either semi-insulating material or p-type material, while the regions on either side are n-type material.
According to a broad aspect of the invention, there is provided a semiconductive double heterostructure optical waveguide coupler comprising: a first layer of semiconductive material having first and second major surfaces; a second layer of semiconductive material on said first major surface; a third layer of semiconductive material on said second major surface, said first layer having an index of refraction higher than said second and third layers; a plurality of channels of semi-insulating material forming coupled optical waveguides, said channels extending through said first and second layers; and means for applying a reverse bias across said first, second and third layers to produce a depletion region which alters the optical coupling between said waveguides.
There follows a description of variable coupling optical waveguide directional couplers embodying the invention in preferred forms. The description refers to the accompanying drawings, in which: